Check valves are used in piping systems to allow the flow of a fluid (a liquid, slurry, gas, or liquid-gas) in one direction but not in the other direction. A ball-type check valve utilizes a ball which is not directly attached to any other component of the valve, but where the ball is constrained within a cage or other supporting assembly. When the fluid flows in the allowed direction, the ball is supported within the cage allowing fluid to flow around the perimeter of the ball. When the fluid flows in the opposite direction, the ball is pressed against a seat, thereby blocking fluid flow in that direction. Unlike flapper-type check valves, which only have one sealing surface, a ball-type check valve can have an infinite number of sealing surfaces created between the ball the seating ring. Because of this feature, if a portion of the ball surface is scratched or otherwise damaged, the ball will rotate until an undamaged surface of the ball finds a seal with the seat.
Among other applications, ball-type check valves may be used where the fluid flowing through the pipe contains abrasive solids. In addition, the flow velocity through the pipeline may be substantial. In such cases the valve sealing surfaces are subject to damage from erosion resulting in valve leakage and failure.
An example of such a hostile application is a production flow-line utilized in transporting hydrocarbons from an oil well to a storage tank. A check valve is required in this application to prevent fluid from the storage tank from draining back into the oil well in the event of a tubing leak or other conditions which could cause a vacuum on the flow-line. Produced fluids from oil wells may contain solids, such as sand and scale. For high volume wells, the produced fluids may also be flowing at significant flow velocities through the flow-line,
Ball and seat valves utilized in a check valve may have the following general structure: (1) valve housing comprising a cylindrical tubular ball cage having a through-bore; (2) an annular seat extending perpendicularly across the flow entry end of the bore; (3) a ball positioned within the bore of the cage; and (4) a transverse ball stop extending across the exit end of the bore to limit the travel of the ball.
Some check valves, particularly those used in oil production flow-lines, pumps, may be subjected to continuous operation with the opening/closing sequence occurring thousands of times in a single day, frequently in a corrosive environment, pumping a fluid which may include abrasive solids. This repeated cycle naturally results in wear and tear as the ball travels within the ball cage and encounters the ball cage surfaces, resulting to ball wear over time.
The seal formed between the ball and seat may be subjected to substantial differential pressures and wear damage to the seal will result in undesirable backflow. A check valve which has increased resistance to damage from abrasive solids is desirable.